Currently when clicking on a tag, and then the encouraging offer ("The cryptography tag has no wiki summary, would you like to help us create it?") you learn that:
Tag wikis can be edited by users with more than 1500 reputation, provided:
They are in the top 20 answerers for this tag
They hold the bronze tag badge for this tag
That is still a pretty small group of people - 6 people out of 1000 after about 2 months. Would it make sense to reduce the reputation cap, e.g. to 200 (more like 70 people)? It seems that early on is the best time to let folks discuss tags in the tag wiki, since tag usage is still in a state of flux and we can still easily shape helpful trends in usage.
Update: I defer to more experience from other sites, but as AviD notes, it is useful to contrast this with other tag-related limits. It seems to me that if you can create new tags (now 150) you should be able to describe why. Community wiki editing is at 100 now, and retagging questions is at 200. Editing other content is 1000.